Since post-tensioning was first used domestically in 1949, the industry has seen many technological advances, including seven-wire, low relaxation strand, improved analysis techniques and design software, extruded sheathing, anchors, and plastic duct systems, as well as the development of pre-packaged, non-bleed grouts. Some of the biggest improvements have been in the area of durability. For example, when some of the earliest un-bonded post-tensioned buildings were about 15 years old, corrosion problems started to surface and it was apparent that some tendon sheathings and coatings could not adequately resist corrosion cause by environmental factors. Improvements in system durability have significantly lengthened the lifespan of post-tensioning systems, which continues to be a major focus in the industry.
Starting in the mid-1970s, the Post-Tensioning Institute (PTI) developed tendon material specifications designed to address the corrosion problems. PTI specified improvements in sheathing, coatings and, in aggressive environments, complete encapsulation of the tendons. Advancements that are more recent have opened new doors for post-tensioning as a long-term, durable design solution.
One method of protecting post-tensioning systems is by encapsulation, which, for un-bonded tendons, evolved into a seamless extruded plastic sheathing around individual steel strands. While this technique protected the tendons along their length, it did not address protection of the anchors.
In the late 1980s, the first generation of encapsulated anchors was introduced. Although this technology allowed for great advancement in durability, it did not completely address encapsulation at construction joints. Later generation anchors offer end-to-end protection of the tendon. Such “end-to-end” protection, however, utilizes removable grease filled end caps disposed about the tail (end) portions of the strand for protection. It is common for the caps to detach and fall off during transportation of the anchors from the manufacturer to the jobsite. There is also the case where the caps are improperly installed in the field. This is problematic, as the ends of the strand can become exposed, and un-encapsulated. Thus, corrosion problems may still exist, especially in the most susceptible areas.
In view of these problems, improved encapsulated anchor devices, systems, and methods are needed.